A continuous alloying treatment furnace for a hot dip zinc coated steel sheet is provided above a zinc pot 2 which applies galvanization to a steel sheet 1, as shown in FIG. 5. That is, above the zinc pot 2, a wiping nozzle 3, a heating furnace 4, a holding furnace 5, and a cooling zone 6 are disposed upward in this order. The steel sheet 1 passing through the zinc pot 2 has its surfaces coated with zinc. After the steel sheet 1 is controlled by the wiping nozzle 3 to have a predetermined weight of coating, it is passed through the alloying treatment furnace comprising the heating furnace 4, holding furnace 5, and cooling zone 6. During this process, alloying of the coated layer is performed. As a means of obtaining a heat cycle for forming an alloy layer of the zinc coated steel sheet, an induction heating type heating furnace is used.
Such a galvanized steel sheet, which has been subjected to alloying treatment, is better inweldability, workability, paintability, and corrosion resistance than an ordinary galvanized steel sheet. Thus, it is used frequently as a steel sheet for household electrical appliances and automobiles.
Alloying treatment of a zinc coating needs to be performed to obtain an iron-zinc alloy layer composition which is ideal, particularly, for ensuring both coating adhesion and press formability at the same time. Coating adhesion and press formability are important quality factors, because the former characteristic prevents powdery peeling of the coated layer, called powdering, during working, while the latter results in an alloy layer with a low sliding resistance, thereby reducing a load during forming. In detail, the surface of the steel sheet after alloying treatment should have a coating composition consisting mainly of a .delta..sub.1 phase while minimizing a .zeta. phase with a high sliding resistance, and a hard, brittle .GAMMA. phase which deteriorates powdering resistance, as shown in FIG. 6.
The constitution of the alloy layer is determined by a heat cycle of heating, holding, and cooling, which have to fulfill the following requirements as shown in FIGS. 7(a) and 7(b):
(1) Heating: Rapid heating for suppressing the .zeta. phase. PA1 (2) Holding: Control of the holding temperature and holding time such that the minimum temperature is T.sub.1 or higher, and the holding time is t.sub.1 or longer, for suppression of the .zeta. phase, and that the maximum temperature is T.sub.2 or lower, and the holding time is t.sub.2 or shorter, for suppression of the .GAMMA. phase. PA1 (3) Cooling: Rapid cooling for suppressing the .zeta. phase. PA1 (1) Involving a method for transforming impedance on a load side, as viewed from a high frequency power source, by an impedance matching apparatus which has a matching transformer provided with intermediate contact points (taps) based on a plurality of turn ratios, and which also has a tap selector device with low inductance and capable of passing a high frequency large current.
It is well known that induction heating is suitable as means of obtaining rapid heating and a highly accurate heating temperature (=holding temperature) among the above requirements. Various induction heaters for alloying have been proposed (e.g., Japanese Unexamined Patent Publication Nos. 294091/92, 228528/92 and 320852/93).
The appropriate holding temperature (T.sub.2 -T.sub.1) and the appropriate holding time (t.sub.2 -t.sub.1) vary with the weight of coating, and also vary with the type of steel of the steel sheet.
FIG. 8 shows an example of circuit configuration of an induction heater.
A material 8 to be heated is passed through a solenoid coil 7, and a high frequency current of a frequency from several kHz to 100 kHz is applied to the solenoid coil 7 to flow eddy currents into the material 8, thereby to heat the material 8. The generated heat distribution and the temperature distribution, in the width direction, of the material 8 to be heated by induction heating vary with the type and width of the material 8 as well as the frequency of induction heating. The oscillation frequency of the source of induction heating is nearly in synchronism with the frequency of a resonance circuit composed of the heating coil and the capacitor. Thus, the frequency of the high frequency current flowing in the heating coil is determined by the capacity of the resonating capacitor and the inductance of the solenoid coil. The inductance of the solenoid coil is determined by its shape and number of turns.
The constitution of an apparatus for induction heating, in a heating furnace, is as shown in FIG. 8, and this apparatus can be replaced by an equivalent circuit shown in FIG. 9. The impedance of the load, as viewed from a power source output side in FIG. 9, is given by the equation (1) based on the inductance L of the coil, the capacity C of the capacitor, and the combined resistance R: EQU Z.apprxeq.(L/CR) (1)
Thus, the impedance of the load, as viewed from the power source output side, varies with the shape and type of the material to be heated, as well as the coil impedance in the high frequency power source circuit and the capacity of the capacitor. The relation of the equation (2) holds for the voltage V, the current I and the impedance Z: EQU V=ZI (2)
The output P of the high frequency power source is given by the equation (3) based on the voltage V, the current I and the power factor cos.theta.: EQU P=VI cos.theta. (3)
Thus, the high frequency power source produces power of the equation (3) from the voltage following the equation (2), and the current.
The voltage and current produced by the high frequency power source have their maximum values determined by the capacity of the power source. Theoutput voltage and output current of the power source have a relationship as shown in FIG. 10. The equation (3) and FIG. 10 show that the output of the power source is maximal at the impedance Za.sub.2, the voltage V=V.sub.max, and the current I=I.sub.max. When the impedance is Z.sub.b, which is greater than Z.sub.a, the maximum value of the output is restricted by the maximum value of voltage, V.sub.max. When the impedance is Z.sub.c, which is less than Z.sub.a, by contrast, the maximum value of the output is restricted by the maximum value of current, I.sub.max.
With the configuration of the induction heater shown in FIG. 8, therefore, the impedance of the load varies when the shape or type of the material to be heated changes. As a result, maximum output of the power source is restricted. This may make it impossible to reach the heating temperature necessary for alloying. Hence, there is need for a method which can perform impedance matching easily and appropriately. There is also need for a galvanized steel sheet alloying system which can give an output close to the maximum value of the output of the power source to variously shaped material to be heated, and which enables alloying treatment to be performed at the necessary heating temperature.